Plasma display devices using PDPs (plasma display panels) have the advantage capable of being thinner and having larger screens. Such plasma display devices display images by utilizing light emission which occurs in the discharge of discharge cells constituting pixels.
FIG. 47 is a circuit diagram showing the structure of a sustain driver in a conventional plasma display device.
As shown in FIG. 47, a sustain driver 600 includes a recovery capacitor C11, a recovery coil L11, switches SW11, SW12, SW21 and SW22, and diodes D11 and D12.
The switch SW11 is connected between a power supply terminal V11 and a node N11, and the switch SW12 is connected between the node N11 and a ground terminal. The power supply terminal V11 is supplied with a sustain voltage Vsus. The node N11 is connected to, e.g., 480 sustain electrodes. A panel capacitance Cp corresponding to all capacitances among the plurality of sustain electrodes and the ground terminal is illustrated in FIG. 47.
The recovery capacitor C11 is connected between a node N13 and the ground terminal. The switch SW21 and the diode D11 are connected in series between the node N13 and a node N12, and the diode 12 and the switch SW22 are connected in series between the nodes N12 and N13. The recovery coil L11 is connected between the nodes N11 and N12.
FIG. 48 is a timing chart showing the operation in a sustain time period of the sustain driver 600 shown in FIG. 47. FIG. 48 shows a voltage at the node N11 and the operation of the switches SW21, SW11, SW22 and SW12.
First, in a time period Ta, the switch SW21 turns on, while the switch SW12 turns off. At this time, the switches SW11 and SW22 are off. Accordingly, LC resonance caused by the recovery coil L11 and the panel capacitance Cp makes the voltage at the node N11 rise to a peak voltage Vp, so that the charges stored in the recovery capacitor C11 are supplied to the panel capacitance Cp. At this time, when the voltage at the node N11 exceeds a discharge starting voltage in a sustain period, sustain discharge is started.
Then, in a time period Tb, the switch SW21 turns off, while the switch SW11 turns on. Accordingly, the node N11 is connected to the power supply terminal V11, so that the voltage at the node N11 rises sharply. The voltage at the node N11 is fixed to the sustain voltage Vsus in a time period Tc.
Then, in a time period Td, the switch SW11 turns off, while the switch SW22 turns on. Accordingly, the LC resonance caused by the recovery coil L11 and the panel capacitance Cp makes the voltage at the node N11 gently drop, so that the charges are recovered from the panel capacitance Cp to the recovery capacitor C11.
Finally, in a time period Te, the switch SW22 turns off, while the switch SW12 turns on. This causes the voltage at the node N11 to sharply drop, so that the voltage is fixed to a ground potential.
Repeating the above operation in the sustain time period causes periodical sustain pulses Psu to be applied to the plurality of sustain electrodes. At the time the sustain pulses Psu rise, a discharge is carried out in discharge cells, and sustain discharge is then made. Further, the charges in the panel capacitance Cp are recovered by the recovery capacitor C11 in a time period Td. The recovered charges are again supplied to the panel capacitance Cp in the time period Ta, thereby accomplishing reduced power consumption.
In the conventional sustain driver, however, power is consumed due to ON resistances of field-effect transistors used as the switches SW21 and SW22, losses caused by the diodes D11 and D12, a DC resistance of the recovery coil L11, resistances of the electrodes forming the panel capacitance Cp, and the like, so that reactive power is produced, in the time periods Ta and Td to be a recovery time.
This reactive power LP is expressed by the following equation where Vsus is the sustain voltage of sustain pulses Psu, Vp is a peak voltage in the recovery time, and F is the number of sustain pulses for one second.LP=Cp×Vsus×(Vsus−Vp)×F
If a longer recovery time is set, then the peak voltage Vp due to LC resonance can be made higher, enabling reduced reactive power; however, if a longer recovery time is set in the case of a high lighting rate, then stable discharge cannot be made. Therefore, a shorter recovery time is set for every lighting rate.
Thus, in the conventional plasma display device, the peak voltage Vp in the recovery time decreases, thereby failing to sufficiently reduce the reactive power and the power consumption in the case of a low lighting rate.